The commercial production of L-threonine has heretofore been conducted by fermentation processes because like other amino acids, L-threonine has stereoisomerism which makes the selective production of its L-isomer difficult. As examples of such fermentation processes, there may be mentioned the processes using amino acid-requiring strains (e.g Japanese Patent Publication Nos. 3319/71, 34193/71 and 34194/71) As to production processes relying on precursor fermentation, the processes using homoserine as a precursor (e.g. Japanese Patent Publication Nos. 2896/61, 6590/63 and 8715/68) can be mentioned.
As enzymatic production processes which are less costly in fixed cost and lend themselves better to production control than fermentation processes, the processes using glycine and acetaldehyde as precursors (e g. Japanese Laid-Open Patent Application Nos. 121491/81 and 116681/83), for instance, have been proposed but none of these processes have been successfully developed into commercial processes because of by-production of allothreonine.
Aside from these processes, it has been reported that L-threonine can be produced in a reaction medium containing DL-homoserine by means of various microbial cells [Shimura et al, Amino Acids, Vol 1, pp. 71-74, published by Association of Amino Acid Fermentation, (1959)]. However, by any of these known methods, the yield of L-threonine is not fully satisfactory.
On the other hand, not much research work for the commercial exploitation of DL-homoserine as a precursor has been reported This is because although the DL-form of homoserine has been chemically synthesized at comparatively low cost, the production of L-threonine requires a very high cost because L-threonine is formed only from L-homoserine but not from D-homoserine (Amino Acids, Vol 1, pp. 71-74, 1959).